Check Mail's preferences under Composing Tab. Check to see what account is the default sending account.
Also, edit the SMTP server list. Since you deleted the comcast account, delete the comcast outgoing server from the list.
Just to confirm:
In Mail preferences, you went to Accounts, found your AT&T account and edited the Outgoing Mail Server (SMTP) list from the little menu thingy? You made sure Comcast was deleted and clicked on the AT&T server and confirmed that the username and password under the Advanced tab for the AT&T outgoing SMTP mail server is correct?